Definition Contact lenses are thin, curved plastic disks designed to cover the cornea, the clear front covering of the eye. Contacts cling to the film of tears over the cornea because of surface tension, the same force that causes a drop of water to cling to the side of a glass. Contacts provide a safe and effective way to correct vision when used with care and proper supervision. They can offer a good alternative to eyeglasses, depending on your eyes and your lifestyle. Over 24 million people in the United States now wear contact lenses.Symptoms Contact lenses are used to correct the same conditions that eyeglasses correct:

Myopia (nearsightedness)

Hyperopia (farsightedness)

Astigmatism (distorted vision)

Presbyopia (need for bifocals)

Special tinted contacts can be used to change the color of the eyes to various degrees. Contact lenses are sometimes used therapeutically in eye diseases where an uneven cornea blurs vision, such as keratoconus or scarring.
The health of your eyes should be your main concern. An eye doctor will help you decide whether contact lenses are right for you. Choose an eye care professional who is knowledgeable and experienced with contact lenses.Types of Contact Lenses
Many types of contact lenses are available. The type of contacts you use depends on your particular situation. Your eye doctor will be able to help you choose from the following types of lenses.
PMMA lenses: Rigid or "hard" contacts were the first lenses; they were developed in the 1960's. They are made of a type of plastic called PMMA (polymethyl methacrylate), which is very durable, but does not allow oxygen in the air to directly reach the cornea. When the eye blinks, the lens moves, which allows the oxygen dissolved in the tears to reach the cornea. Rigid lenses are the least comfortable type of contacts and are not really used anymore. However, some people still prefer them for their durability and lower cost.
Gas-permeable lenses: These lenses are also known as "RGPs." They are newer rigid or "hard" lenses made of plastics combined with other materials, such as silicone and fluoropolymers, which allow oxygen in the air to pass directly through the lens. For this reason, they are called "gas permeable."
Soft contact lenses: These lenses are made of plastic materials that incorporate water. The water makes them soft and flexible, as well as allowing oxygen to reach the cornea. More than 75% of contact lens wearers in the United States use soft lenses.

Extended wear contact lenses: made of material designed to last 2-4 weeks.

Toric contact lenses: correct moderate astigmatism. They are available in both rigid and soft materials.

Generally, contacts should be removed at bedtime due to risk of infection and risk of contact lens intolerance.What is the Cost? When comparing the price of contact lenses, it's important to consider what services are included. Does the fitting include a thorough eye examination and follow-up? Can you exchange lenses during the initial fitting, and is insurance for lost lenses available? If you need treatment for an eye condition not directly related to the contact lenses, such as inflamed eyelids or dry eyes, there may be additional charges.What Are the Risks? PMMA lenses are more likely to scratch the cornea if the lens does not fit properly or if the lens is worn while sleeping. They are also more likely to slide off the cornea and become hidden under the lid. Rigid lenses traditionally had a reputation for "popping out" of the eye. New lens designs have minimized the chance of loosing a contact even during vigorous exercise. Rigid gas-permeable lenses and soft extended-wear contacts are the most likely to have protein build-up and cause lens-related allergies. Protein build-up results in discomfort, blurring and intolerance to the lenses. You will need special cleaning solutions to dissolve the protein.
Daily-wear lenses should never be worn as extended-wear lenses. Misuse can lead to temporary and even permanent damage to the cornea. People who wear any type of lens overnight have a greater chance of developing infections of the cornea. These infections are often due to poor cleaning and lens care. Improper over wearing of contact lenses can result in intolerance, leading to the inability to wear contact lenses.
Rigid gas-permeable or disposable lenses may be good choices for someone with allergies.Who Should NOT Wear Contact Lenses? Most people who need vision correction can wear contact lenses, but there are some exceptions. Some of the conditions that might keep you from wearing contact lenses are: frequent eye infections, severe allergies, dry eye (improper tear film), a work environment that is very dusty or dirty and inability to handle and care for the lenses properly.Are Contacts For You? Whether or not contact lenses are a good choice for you depends on:

individual needs and expectations

patience and motivation during the initial adjustment period to contact lens wear.

adhering to contact lens guidelines for wear, disinfecting and cleaning

diagnosis and treatment of conditions that may prevent contact lens wear